The present invention relates to an optical telecommunication system, and, more particularly, to an L-band erbium doped fiber amplifier (EDFA) employed therein.
Recently, a various transmission techniques have been developed to transmit a large amount of transmission information. One of the techniques is a wavelength division multiplexing (WDM) in which a number of channels having different wavelengths are simultaneously transmitted in a single optical fiber to increase the transmission capacity.
One of components in a transmission system using the WDM technique is an optical amplifier (OA) that amplifies an optical signal without converting from an optical signal to an electrical signal. This optical amplifier easily compensates an optical fiber transmission loss and/or a distribution network loss. Especially, as the optical amplifier, an erbium doped fiber amplifier (EDFA) is widely employed.
Since the EDFA amplifies the optical signal with a high gain at wavelength ranging from 1.52 xcexcm to 1.63 xcexcm wherein a transmission loss of an optical fiber is smallest, an optical transmission system utilizes the EDFA as an optical amplifier. In particular, the EDFA can amplify different wavelength optical signals at one time so that it is very useful at the WDM technique.
Generally, the EDFA comprises an erbium doped fiber (EDF) that serves as an optical amplification medium, a pumping light (PL) for pumping the doped erbium ions in the EDF, a WDM coupler for multiplexing the signal light and the pumping light, and an optical isolator (ISO) for passing a forwardly progressing optical light and blocking a backwardly progressing optical light by reflection. When the pumping light from the WDM coupler is inputted to the EDF, the erbium ions are pumped. While the inputted signal progresses along the EDF, the signal light is amplified and then the pumped erbium ions are transited to a low energy stage. The characteristic of the EDFA is represented as a gain and a noise figure.
The gain and the noise are largely influenced by the wavelength of the pumping light and the structure of the pump. The general wavelength of the pumping light is 980 nm or 1,480 nm.
The pumping method is classified according to the direction of the signal light and the pumping light. In case that the directions of both signal and pumping lights are identical, this is called as a forward pumping. In case that the directions of both signal and pumping lights are not identical, this is called as a backward pumping. When the forward and the backward pumping are employed together, this is called as a bi-directional pump. In general, the forward pumping is effective in reducing the noise figure, while the backward pumping is effective in increasing the power conversion efficiency (PCE). Therefore, the pumping method is important to design the EDFA.
EDFA is divided as a conventional band (C-band) EDFA amplifying the signal of 1,550 nm wavelength band, and a long band (L-band) EDFA amplifying the signal of 1,580 nm wavelength band. By advent of the L-band EDFA, optical communication bandwidth is expanded.
In a prior art, the pumping light is produced by a laser diode. The EDFA is constructed by fusing-coupling two optical fibers instead of the WDM coupler. In one application, the EDFA is used for a fiber ring laser (see, U.S. Pat. No. 4,955,025). As one of the pumping methods, a bi-directional pumping to forward pump by a 980 nm laser diode (LD) and to backward pump by a 1,480 nm LD is introduced to reduce a noise figure and increase a PCE of C-band EDFA (see, U.S. Pat. No. 5,140,456).
As another prior art, an L-band EDFA using a 980 nm LD as main pump and 1,550 nm DBF LD as a subsidiary pump with small power is provided to enhance the PCE at 1,570xcx9c1,600 nm wavelength band (See, R. Di Muro, N. E. Jolley, J. Mun. xe2x80x9cMeasurement of the Quantum Efficiency of long wavelength EDFAs with and without an idler signalxe2x80x9d ECOC""98 technical digest, 20-24pp., Sep., 1998).
Hitherto, 980 nm and 1,480 nm wavelength are used as pump wavelengths for the L-band EDFA but the EDFA pumped by 1,530 nm wavelength band pump is not introduced.
It is, therefore, a primary object of the invention to present a 1,530 nm band pumped EDFA enabling L-band (1,570xcx9c1,600 nm) input light to be amplified by the 1,530 nm band pump, thereby increasing the PCE. The increase of the PCE means that the efficiency of pump power of the EDFA is enhanced.
It is another object of the invention to provide a two-stage amplifying structure using 1,530 nm band pump, thereby reducing a noise characteristic and increase a gain characteristic.
To achieve the above object, in one embodiment of the present invention, an L-band EDFA comprises: an input terminal for receiving signal light; a pumping unit for pumping a 1,530 nm wavelength band pumping light; a WDM coupler for multiplexing the signal light and the pumping light; and an EDF for amplifying the signal light by the pumping light; isolators for removing a reflection noise, each of which is located between the first amplifying stage and the second amplifying stage, and between the second amplifying stage and an output terminal.
The pumping unit on this embodiment has a first amplifying stage and a second amplifying stage. The first stage comprises a tunable light source that has a capability to change the wavelength of the pumping light, 980 nm wavelength laser diode for producing 980 nm wavelength forward pumping light, a first WDM coupler for multiplexing the output light from the tunable light source and the output light from the 980 nm wavelength laser diode, and a first EDF for amplifying the 1,530 nm band light provided from the first WDM coupler. The second amplifying stage comprises a first 1,480 nm wavelength LD for producing 1,480 nm wavelength forward pumping light, a second WDM coupler for multiplexing the output light from the first stage and the pumping light from the first 1,480 nm wavelength LD, a second 1,480 nm wavelength LD for producing 1,480 nm wavelength backward pumping light, a third WDM coupler for multiplexing the output light from the second 1,480 nm wavelength LD and the output light from the second WDM coupler, a second EDF for amplifying the 1,530 nm band light provided from the first amplifying stage, and an optical tunable filter for reducing the amplified spontaneous emission noise light from the amplified light at the second erbium doped fiber. In addition,
To achieve the above object, in another embodiment of the present invention, an optical amplifier comprises: an input terminal for receiving an L-band signal light; a 980 nm LD for forward pumping 980 nm wavelength pumping light; a first WDM coupler for multiplexing the signal light and the pumping light; a 1,530 nm band LD for backward pumping 1,530 nm wavelength pumping light; a second WDM coupler for multiplexing the signal light and the 1,530 nm pumping light; a EDF for amplifying the signal light by bi-directional pumping lights; and two isolators to remove a reflection noise, each of which is located between the input terminal and the first wavelength division multiplexing unit, and coupled to an output terminal.
To achieve the above object, in still another embodiment of the present invention, an optical amplifier comprises: a first stage having an input terminal to which a signal light is inputted, 980 nm LD for forward pumping 980 nm wavelength pumping light, a first WDM coupler for multiplexing the signal light and the 980 nm wavelength pumping light, and a first EDF for amplifying the signal light provided from the first WDM coupler; and a said second amplifying stage having a first 1,530 nm LD for forward pumping 1,530 nm wavelength band pumping light, a second WDM coupler for multiplexing the output light from the first EDF and the pumping light from the first 1,530 nm LD, a second 1,530 nm LD for backward pumping 1,530 nm wavelength band pumping light, a third WDM coupler for multiplexing output light from the second WDM coupler and the output light from the second 1,530 nm LD, and a second EDF for amplifying the signal light from the first amplified stage by bi-directional pumping, wherein, said amplifier further comprises two isolators to remove a reflection noise, each of which is located between the first amplifying stage and the second amplifying stage, and between the second amplifying stage and an output terminal